The worldwide increasing demand for light olefins remains a major challenge for many integrated refineries. In particular, the production of some valuable light olefins such as ethylene, propylene, and butylenes has attracted increased attention as pure olefin streams are considered the building blocks for polymer synthesis. The production of light olefins depends on several process variables like the feed type, operating conditions, and the type of catalyst. Despite the options available for producing a higher yield of propylene and light olefins, intense research activity in this field is still being conducted. These options include the use of HSFCC systems, developing more selective catalysts for the process, and enhancing the configuration of the process in favor of more advantageous setting.
The HSFCC process is capable of producing yields of propylene up to four times higher than the traditional fluid catalytic cracking unit and higher conversion levels for a range of petroleum steams. That being said, achieving maximum propylene and conversion from a wide range of feed qualities offers considerable challenges to the catalyst design for the HSFCC.
Moreover, the conventional FCC feedstocks range from hydrocracked bottoms to heavy feed fractions such as vacuum gas oil and atmospheric residue. However, these feedstocks are limited, obtained through costly and energy intensive refining steps, and thus are not expected to fulfill the ever growing market demands.
The addition of zeolites to an HSFCC catalyst is utilized for improving the yield of light olefins due to its shape selectivity, special pore structure and large specific surface area. However, when the crystal size of the zeolites is close to the molecular diameter of light hydrocarbons, the diffusion of the reactant/product molecules within the micropores is usually the rate-limiting step of the reaction. Furthermore, the crystal surface of the zeolites are susceptible to coke formation, which obstructs the accessibility of the micropores and thus deactivates the catalyst.